The system for purifying the exhaust gas of an internal combustion engine (hereinafter also referred to simply as the engine) is primarily comprised of a catalytic converter and an air-fuel ratio feedback apparatus. The catalytic converter is disposed in an exhaust pipe for eliminating HC (hydro-carbon), NOx (oxides of nitrogen) and CO (carbon monoxide) contained in the exhaust gas discharged from the engine. On the other hand, the air-fuel ratio feedback control apparatus is installed for controlling the amount of fuel injected to the engine by making use of an output signal of an oxygen sensor disposed in the exhaust pipe upstream of the catalytic converter for thereby maintaining the air-fuel ratio essentially at a constant level because the air-fuel ratio has to be held constant in order that the catalytic converter works most efficiently. In the case of a three-way catalyst system, degradation in the performance in the oxygen sensor disposed upstream of the catalytic converter causes the air-fuel ratio to deviate from a narrow range predetermined around a center value given by the stoichiometric air-fuel ratio, as a result of which the conversion efficiency of the catalytic converter for converting the HC, CO and NOx components of the exhaust gas to harmless components undergoes degradation. Besides, deterioration of the performance of the catalytic converter itself will be accompanied with degradation in the efficiency of conversion even when the air-fuel ratio is controlled accurately. As a technique for diagnosing the catalytic converter as to whether it suffers deterioration in the conversion performance or capability, there may be mentioned a diagnosis system disclosed, for example, in JP-A-5-171924 and the corresponding U.S. Pat. No. 5,341,642. More specifically, this publication disclose an apparatus for diagnosing an engine exhaust gas purifying system, which is comprised of an upstream air-fuel ratio sensor for detecting an air-fuel ratio at a location upstream of a catalytic converter as viewed in the exhaust gas flowing direction, a downstream air-fuel ratio sensor for detecting an air-fuel ratio within the exhaust pipe at a location downstream of the catalytic converter, a characteristic waveform extracting means for attenuating signal components of a frequency band which is lower than an air fuel ratio control signal frequency band of an air-fuel ratio control means in the output signals of the upstream air-fuel ratio sensor and the downstream air-fuel ratio sensor, a means for arithmetically determining a correlation function of the signal passed through the characteristic waveform extracting means, and a catalyst state decision means for deciding the state of deterioration of the catalytic converter on the basis of values of the correlation function.
The performance of the catalytic converter is susceptible to the influence exerted by the temperature of the catalyst. When this temperature is low, the conversion efficiency becomes lowered. Accordingly, unless the temperature of the catalyst is taken into consideration in diagnosing the performance of the catalytic converter, an erroneous or false diagnosis will undesirably be resulted because lowering of the conversion efficiency of the catalytic converter under the influence of a change in the temperature thereof which is affected by the engine speed (rpm) and engine operation state such as an engine load level may erroneously be decided as deterioration in the performance of the catalytic converter. For solving this problem, it is required to install a temperature sensor for correcting the result of the diagnosis, as is disclosed in JP-A-5-171924 and corresponding U.S. Pat. No. 5,341,642. However, the catalyst temperature sensor mentioned above has to be implemented so as to cover a very wide range of temperatures for measurement on the order of 0xc2x0 C. to 800xc2x0 C., which involves high expensiveness in manufacturing the diagnosis apparatus, giving rise to another problem.
The problem mentioned just above may be solved by estimating the temperature of the catalytic converter on the basis of a signal measured for other purpose in place of the installing the catalyst temperature sensor. In this conjunction, it should however be mentioned that the means for allowing the catalyst temperature to be measured with high or satisfactory accuracy required for diagnosing the catalytic converter as to deterioration. in the performance thereof has been neither proposed nor realized heretofore.
In the light of the state of the art described above, it is an object of the present invention to provide a method and an apparatus which are capable of diagnosing deterioration of a catalytic converter which constitutes a major part of an engine exhaust gas purification system without fail by estimating accurately the catalyst temperature in the course of normal running of a motor vehicle.
It is an object of the present invention to provide a method and an apparatus for diagnosing an engine exhaust gas purification system which are capable of diagnosing with high accuracy the catalytic converter as to occurrence of deterioration in the performance thereof by estimating the catalyst temperature in the course of running an internal combustion engine.
In view of the above and other objects which will become apparent as the description proceeds, it is proposed according to a general aspect of present invention that a plurality of engine operation parameters are detected, either a temperature of a catalyst or temperature of an engine exhaust gas at a location in the vicinity of the catalyst is determined on the basis of the engine operation parameters, and that deterioration of the catalyst is decided by using as indexes the catalyst temperature or the exhaust gas temperature in the vicinity of the catalyst.
In a preferred mode for carrying out the present invention, there is provided a unit for estimating the temperature of the catalyst in a steady state by making use of an engine speed (rpm) and an engine load which represent typical parameters employed in the fuel injection control of the internal combustion engine. The steady-state temperature estimating unit mentioned above can easily be realized by storing the engine speeds (rpm) and the engine loads obtained experimentally by operating the engine in the steady state in the form of a two-dimensional data map. Additionally, a unit for estimating the temperature of the catalyst in the transient state of the engine is provided for correcting the catalyst temperature determined by the steady-state catalyst temperature estimating unit for thereby ensuring correct estimation of the catalyst temperature even in the course of operation of the motor vehicle equipped with the engine. In the transient temperature estimating unit mentioned above, non-linear characteristics are each described by combining a plurality of linear characteristics. Further provided is a unit for estimating the temperature of the catalyst on the basis of the value determined by the steady-state temperature estimating unit and the transient-state temperature estimating unit for thereby deciding whether or not the catalyst suffers deterioration. With the arrangement described above, there can be realized a diagnosing apparatus which is capable of diagnosing with high accuracy and reliability the catalyst of the engine exhaust gas purification system as to whether or not the catalyst suffers deterioration in the performance or capability thereof.
In the diagnosis apparatus for the engine exhaust gas purification system mentioned above, it is preferred to provide a dead time setting unit for taking into account a temperature rise due to latent heat of moisture resident in the catalyst and associated piping at the time of starting the engine for thereby increasing the accuracy of the estimation of the catalyst temperature as performed on the basis of the outputs of the steady-state catalyst temperature estimating unit and the transient-state catalyst temperature estimating unit.
Furthermore, it is preferred to provide in association with the transient-state catalyst temperature estimating unit a facility for changing over delay time constants between a catalyst temperature rising process in which the catalyst is heated by the exhaust gas and a catalyst temperature lowering process in which the catalyst heat is dissipated to the atmosphere.
In another preferred mode for carrying out the invention, the diagnosis apparatus for the engine exhaust gas purification system should include a unit for correcting the estimated catalyst temperature in dependence on change or changes in the engine cooling water temperature, intake air temperature and/or the vehicle speed.
In a process for executing the catalyst deterioration diagnosis according to the invention, diagnosis of the catalyst itself is performed. To this end, a method disclosed in U.S. Pat. No. 5,341,642 may be utilized.
More specifically, by providing an upstream air-fuel ratio sensor at a position upstream of the catalyst and a downstream air-fuel ratio sensor disposed downstream of the catalyst, the air-fuel ratios are detected at locations upstream and downstream of the catalyst, respectively, under the timing determined by a crank angle signal generated by an crank angle detector which may be conventional one usually provided in the engine. In the output air-fuel ratio signals thus obtained, those signal components which falls within a lower frequency band than that employed for the air-fuel ratio control are attenuated by a characteristic waveform extracting unit.
On the other hand, an autocorrelation function calculating unit arithmetically determines an autocorrelation function xcfx86xx of the signal passed through the characteristic waveform extracting unit, while a cross-correlation function calculating unit determines a cross-correlation function xcfx86xy between the output signal of the upstream air-fuel ratio sensor and the output signal of the downstream air-fuel ratio sensor, both of which signals have passed through the characteristic waveform extracting unit mentioned above.
A deterioration index calculating unit determines a ratio between a maximum value (xcfx86xy)max of the cross-correlation function xcfx86xy and the maximum value (xcfx86xx)max of the autocorrelation function, respectively, during every predetermined period, to thereby output successive or sequential deterioration indexes "PHgr"i. By determining a mean value from a predetermined number of the sequential deterioration indexes "PHgr"i calculated in this manner, there is outputted an ultimate deterioration index represented by the mean value.
In a preferred mode for carrying out the invention, the engine speed (rpm) and the intake air flow are detected to thereby determine the temperature of the catalyst when the engine operation state is stable (i.e., the engine speed and the engine load are in steady state). On the other hand, the transient-state catalyst temperature estimating unit determines the catalyst temperature in dependence on the engine operation states through cooperation with the dead time setting unit for taking into account the temperature rise at the start of engine operation as mentioned previously. Subsequently, the estimated catalyst temperature is corrected by taking into account the changes in the engine cooling water temperature, intake air temperature and the vehicle speed by the estimated catalyst temperature correcting unit. In this manner, the temperature of the catalyst can be estimated or determined with high accuracy without need for providing a sensor destined for this end.
When it is decided by a catalyst temperature comparison unit that the estimated catalyst temperature determined in this way exceeds a predetermined value, a deterioration indicating index is outputted as the result of the diagnosis.